This invention pertains to digital filtering and more particular to multistage filtering based on more than one criterion.
In many applications, a digitizing tablet is used to convert the pen or stylus motion into a set of electrical data which can be processed by digital equipment. Typically, there would be a special electronic or electromagnetic surface which is able to detect the X-Y position of the pen at a periodic rate. This information is present as two digital data words at a periodic clock rate. This class of technology is used for signature verification, automatic drafting, character recognition, etc. In each case, the user writes with the electronic pen and the position is monitored electronically.
In connecting such a device directly to a recognition processor there are problems because the raw data contains cetain kinds of noise and other defects. The raw data contain noise which must be minimized by smoothing or filtering. In this problem, however, there are two different kinds of noise. There is the usual electrical noise such that a stationary pen will not give the same value each time due of random fluctuations in the sensing medium. Secondly, there is a "natural" noise produced by the human writer when he is writing slowly. If one looks at a human drawn line with a magnifying glass, one sees that the line contains many wiggles. This is especially true for slowly drawn lines.
When a line is drawn slowly, there is a very large amount of data even though there is very little information. It is very desirable and often required to perform data reduction. Consider a system which samples the x-y values at a rate of 120 per second in order to follow very fast motion. A given line written slowly could generate from 50 to 100 points whereas the same line written rapidly could generate only 10 to 20 points.